In northwest Belgium’s town of Ghent, one of the oldest and most secure medieval castles is still standing. Constructed nearly 850 years ago, Gravensteen castle — or the “Castle of the Counts” — is an imposing structure, built entirely from stone and complete with a whopping 24 towers, a massive moat, and a defense system that’s still largely intact today. With such protection tactics in place, the Castle of the Counts enjoyed a lengthy history free from siege for nearly eight centuries. In fact, the very first occupation of Gravensteen didn’t occur until 1949, and it wasn’t military forces that laid the siege — it was college students, and it was all thanks to a 1-franc increase on beer.

On the morning of Nov. 16, 1949, 138 students from the University of Ghent took to the castle walls to protest their outrage over a new Belgian beer tax that brought the cost of beer from 3 francs to 4. They started by lowering the portcullis, capturing the only guard on duty and locking him in a closet before barricading the door with a cart filled with rotting fruit. The students then raised banners above the castle walls, demanding that the price of beer in the city be lowered back to three francs.

While the first few hours of the invasion were relatively uneventful, the students got the action they were looking for when two police officers biked by. As the officers passed the castle, the students took to the cart they brought along with them and pelted the cops with rancid fruit in protest. The officers called in backup, and shortly after, a brigade of police officers and firemen arrived at the scene and were met with more fruit-throwing.

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Eventually — and relatively easily — firefighters were able to get a ladder up and over the castle walls to detain the students. However, following such an outpouring of public support from the surrounding Ghent community, not one of the 138 students ended up facing prosecution for the hours-long occupation. Despite their valiant efforts, the students were unsuccessful in reinstating the 3-franc beer.

Today, the Battle of Gravensteen Castle is fondly remembered as the greatest student prank in history by the Ghent community. Every Nov. 16, the “battle” is commemorated by University of Ghent students who pour into the streets surrounding the castle to dance, sing “The Battle of Gravensteen” (a song written about the protest) and, of course, drink beer.